The present invention relates to a process for preparing polyester resins. According to the process a carboxylic acid or corresponding anhydride containing ethylenic unsaturation is first reacted with a saturated, monohydric alcohol to form the half ester of the acid or anhydride. The half ester is then reacted with a polyol to form the polyester.
In general unsaturated polyesters are prepared from dicarboxylic functional monomers, or mixture of di- or greater carboxyl functional monomers where at least one of which contains ethylenic unsaturation. These polyesters are obtained by the condensation of the carboxylic acid monomers with polyhydric alcohols. Commercially, the polyester is dissolved in a monomer such as styrene to obtain a solution that may then be crosslinked.
U.S. Pat. No. 2,478,015 discloses a process for the preparation of unsaturated polyesters by reacting the monoester of a beta-unsaturated monohydric alcohol and polycarboxylic acid with a polyhydric alcohol. Examples of the unsaturated alcohols include allyl alcohol, methallyl alcohol, crotyl alcohol, etc. Beta-unsaturated alcohols such as allyl alcohol are toxic and esters prepared using such alcohols have a tendency to gel. U.S. Pat. No. 3,547,898 discloses a process for the production of resins containing maleic half esters. In this process maleic acid or anhydride is reacted with a monoalkyl or alkaryl ethoxylate having from 4 to 15 carbon atoms in the alkyl or aralkyl group. An alcohol such as methanol may also be added to the reaction mixture. The reaction product is then dissolved in a monomer with a free radical initiator aralkyl group. An alcohol such as methanol may also be added to the reaction mixture. The reaction product is then dissolved in a monomer with a free radical initiator present. U.S. Pat. No. 3,979,443 discloses the preparation of monohydric alcohol esters of maleic acid. U.S. Pat. No. 5,118,783 discloses a process for preparing low molecular weight unsaturated polyesters by adding a monofunctional alcohol to the condensation reaction of dicarboxylic acids and polyhydric alcohol. Japanese Patent No. 2,863,896 discloses a process for the preparation of an ester derivative useful as a cross linking improver for polymers. The abstract of the '896 patent discloses reacting maleic anhydride with isopropanol, isomerizing the reaction product to give monoisopropyl fumarate and reacting that fumarate with a bisphenol A derivative at a temperature of 120.degree..
Styrene, a solvent used to prepare solutions of unsaturated polyesters is considered a hazardous pollutant. Much work has been done in an effort to prepare low volatile organic compound (VOC) unsaturated polyester resin systems. One area of focus has been the use of waxes as a means of reducing emissions. During cure waxes which are initially dissolved or dispersed in a resin, form a thin film on the surface of the fabricated article. The film acts as a physical barrier preventing styrene from evaporating from the surface of the curing part. This film reduces styrene emissions. Unfortunately the waxy film substantially diminishes interlaminar adhesion thus reducing the strength of molded articles made using a multilaminate construction. An alternative to the use of waxes is the use of low molecular weight unsaturated polyester resins. The lower molecular weight permits the use of less styrene because of increased solubility of the resin in the styrene. Typically the molecular weight of unsaturated polyester resins is manipulated by altering the ratios of components. The highest molecular weight is achieved when a 1:1 ratio of acid to polyol is used. Increasing the ratio of one component in relation to the other lowers the molecular weight. However, these synthesis techniques significantly change the performance characteristics of the resulting products. For example, using more polyol than acid can significantly reduce the reactivity and/or thermal resistance of a product. Using more acid than polyol will increase the reactivity but the acid value (AV) of the product also increases. High AV resins are not very soluble in styrene and tend to precipitate in styrene. Articles molded from these resins are susceptible to attack by water. Simply changing molecular weight without modifying the nature of the end groups diminishes the suitability of the resins for many applications.
Unlike standard methods for the preparation of unsaturated polyester resins the invention provides a method of preparation yielding resins having both a low viscosity and acid value. Resins prepared according to the invention are more soluble in styrene while still retaining good performance characteristics. Because the resins are more soluble less styrene is necessary to prepare a styrene resin solution. Reducing the styrene content allows manufacturers to continue to use conventional open mold techniques while reducing styrene emissions.